The etiology and pathogenesis of temporomandibular joint disorders, including rheumatoid arthritis (RA) and temporomandibular disorders, are poorly understood. We have found that artificial elevation of peripheral glutamate levels in masseter muscle or TMJ tissues evokes nociceptive afferent discharges and jaw reflexes involving brainstem subnucleus caudalis in rats and induces pain in humans; sex-related differences occur in the responses to glutamate. This has led to the general hypothesis that activation of peripheral NMDA receptors may contribute to masseter muscle or TMJ pain and the female predominance in TMJ disorders. To address this, we will test (A) if rat nociceptive masseter or TMJ afferent fibers can be excited by glutamate application to masseter or TMJ that is mediated through activation of peripheral NMDA receptors and blocked by peripherally applied NMDA receptor antagonist, and if there are sex-related differences in these effects that are dependent on sex steroid hormones; (B) if glutamate levels in rat masseter or TMJ are elevated by excitation of masseter or TMJ afferents, and if there are sex-related differences in these levels that are dependent on sex steroid hormones; (C) if the excitability of rat caudalis nociceptive neurons is enhanced by glutamate-induced activation of peripheral NMDA receptors in masseter or TMJ and blocked by peripherally applied NMDA receptor antagonist, and if there are sex-related differences in this excitability increase that are dependent on sex steroid hormones; (D) if injection of glutamate into the human masseter or TMJ evokes pain in humans through activation of peripheral NMDA receptors, and if there are sex-related differences in this effect and in the efficacy of peripheral NMDA receptor antagonist to block glutamate-induced pain in these healthy subjects as well as ongoing clinical pain in RA or myofascial TMD patients that are dependent on sex steroid hormones; and (E)) if there is a difference in human masseter or TMJ glutamate levels between healthy subjects and the patients, and if there are sex-related differences in peripheral glutamate levels that are dependent on sex steroid hormones. Insights gained by our multidisciplinary investigations will lead to a better understanding of musculoskeletal pain mechanisms and could provide a basis for the development of novel approaches for the management of pain in TMJ disorders.